A stupid astro photography question!

Hello - this is probably a silly question but, just say I'm taking some sub frames and the clouds roll in (hard to imagine I know) and I've only managed say 7x300sec frames - Is there any benefit in copying those 7 frames say twice and ending up with 21 Subs? I assume there isn't as everyone would be at it but can someone explain why?

Thanks!

If you take one image and simply stack it onto itself multiple times, the stacked image will be no different than the single original.

The idea of taking "sub" images is that when you have many, stacking and processing software can pick out the best bits of all of them and result in a single image with all those "best" bit combined. If the "subs" are all the same image though (or set of images) then there is no "difference" to be picked out.

Perhaps others here can give a better explanation but the above may satisfy you.

Cheers
Dave

p.s.

No such thing as a stupid question - only a stupid answer

Thanks Dave - I take that point about how more unique subs means less noise etc. but the part I can't get my head around is that there is still exposure data in the copied sub frame so if someone says they have 7x300sec subs they would say they had 35mins exposure time so by copying a range of subs, say the same 7 copied x 3 =21 - would that not result in 21x300sec or 1hr 45min exposure??

Assuming you had perfect alignment with guiding and the temperature was regulated on the camera, the differences between the subs must be pretty small anyway particularly when calibrated with darks, flats and bias??

These are the type of thoughts that run through my head at 4 in the morning waiting for my darks to finish!!

Out of curiosity regarding the same question, about two years ago I tried to stack ten copies of a single frame to compare to stacking ten different subs. The result was terrible and very noisey compared to the ten different subs. It seemed to amplify the noise-possibly as it had no reference frames to identify noise and subtract it from.
Good question!

stang wrote: there is still exposure data in the copied sub frame

But it's exactly the same data.

So let's say we take one image and simply copy it. ImageA1 and ImageA2. When we stack them we simply amplify each pixel by exactly the same amount (doubled*).

Alternatively, if we take two images (ImageA and ImageB) and stack them, each pixel is not simply doubled, because they are two different images. Now we get a stacked imaged that is a combination of two images. With correct processing during the "combine" process, we can bring out the best of each image and minimise the worst of each. The more subs we add, the better the whole becomes.

I'm not an expert in this field but have read enough around it. Others may be able to describe it in a more technically correct manner (Nyquist etc.)but I *think* that's the layman's version.

* 'doubled' is likely not the correct terminology, see above...